20 INSECTS OF ECONOMIC IMPORTANCE 



decoction. Unfortunately, the nicotine content of 

 tobacco varies with the variety, soil, method of cur- 

 ing and other factors ^" and it is therefore impos- 

 sible, without chemical analysis, to make a standard 

 decoction. In general, i pound of stems in a gallon 

 of water brought to the boiling point and when 

 drained off enough water added to make a full gal- 

 lon will give a decoction of sufficient strength to 

 kill plant lice. It is best to rely on commercial ex- 

 tracts whose nicotine contents are known. 



Commercial tobacco extracts — Extracts of 

 tobacco are now manufactured commercially and 

 are known as nicotine sulphate. Nicotine sulphate 

 appears to kill insects by its vapor. *^ The most fa- 

 miliar brand is known as black-leaf-40. In this 

 brand the nicotine is combined with an acid and 

 hence is not easily volatile. It contains 40% nico- 

 tine and is readily miscible with water. It is usually 

 used at the rate of i gallon to 800 or 1000 gallons 

 of water. It can be combined with lime-sulphur, 

 and arsenate of lead or wnth Bordeaux mixture.*^ 

 \^^hen the nicotine sulphate is used alone, soap is 

 usually added at the rate of 4 or 5 pounds to 100 

 gallons of water to increase its spreading power and 

 to liberate the nicotine. ^^ A precipitate is some- 

 times formed when nicotine sulphate is added to 

 other mixtures especially w^hen in a concentrated 

 form. 



Soaps 



44 



Ordinary laundry soap, i pound to 5 or 6 gallons 

 of water is a good insecticide for the garden, lawn, 



40 Ellet and Grissom— Vir. Expt. Stat., Bull 208. 



41 Mclndoo— U. S. Dept. Agr., Jr. Agr. Res., Vol. Vll, p. 89. 



42 Headlee — New Jersey Expt. Stat., Rept. Ent. for 1914, p. 356. 



43 Moore and Graham— U. S. Dept. Agr., Jr. Agr. Res., Vol. X, 

 p. 47. 



44 Marlatt— U. S. Dept. Agr., Ear's' Bull. 127, p. 15. 



